Abstract

Since the origin of the high interindividual variability of the chromatic difference in retinal image magnification (CDM) in the human eye is not well understood, optical parameters that might determine its magnitude were studied in 21 healthy subjects with ages ranging from 21 to 58 years. Two psychophysical procedures were used to quantify CDM. They produced highly correlated results. First, a red and a blue square, presented on a black screen, had to be matched in size by the subjects with their right eyes. Second, a filled red and blue square, flickering on top of each other at 2 Hz, had to be adjusted in perceived brightness and then in size to minimize the impression of flicker. CDM varied widely among subjects from 0.0% to 3.6%. Biometric ocular parameters were measured with low coherence interferometry and crystalline lens tilt and decentration with a custom-built Purkinjemeter. Correlations were studied between CDM and corneal power, anterior chamber depth, lens thickness, lens tilt and lens decentration, and vitreous chamber depths. Lens thickness was found significantly correlated with CDM and accounted for 64% of its variance. Vertical lens tilt and decentration were also significantly correlated. It was also found that CDM increased by 3.5% per year, and part of this change can be attributed to the age-related increase in lens thickness.

Stimulus to minimize the perceived brightness differences between red and blue and to quantify perceived differences in size of the filled red and blue squares. The filled squares were presented on top of each other in an alternating fashion, being replaced at 2 Hz. The black cross in the center provides a fixation point. Both brightness and size could be adjusted by the subjects until luminance flicker was minimized and until the perceived sizes were matched.

Psychometric function showing the percentage of correct responses of 11 subjects judging the size differences between two gray squares. Data are from 11 subjects, but fewer data points may be visible because they may be superimposed.

Perceived differences in the size of the red and blue squares, measured in percent, when the red square was brighter (abscissa; red pixel value 255, blue 200) or the blue square was brighter (ordinate; red 200, blue 255). The perceived differences in size were highly correlated. Data points represent the means and their SDs from four repetitions.

Effects of spectacle corrections on the perceived magnification difference of the red and the blue squares. A, All subjects; B, only subjects wearing spectacles (filled circles) or contact lenses (filled triangles). Spectacle lens power was not an important factor to determine CDM. Data are from right eyes.